Re: [PATCH v2] memcg: allow exiting tasks to write back data to swap

[Yosry Ahmed <yosryahmed@google.com>]

On Thu, Dec 12, 2024 at 1:35 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>
> On Thu, Dec 12, 2024 at 01:30:12PM -0500, Johannes Weiner wrote:
> > On Thu, Dec 12, 2024 at 09:06:25AM -0800, Yosry Ahmed wrote:
> > > On Thu, Dec 12, 2024 at 8:58 AM Rik van Riel <riel@surriel.com> wrote:
> > > >
> > > > A task already in exit can get stuck trying to allocate pages, if its
> > > > cgroup is at the memory.max limit, the cgroup is using zswap, but
> > > > zswap writeback is enabled, and the remaining memory in the cgroup is
> > > > not compressible.
> > > >
> > > > This seems like an unlikely confluence of events, but it can happen
> > > > quite easily if a cgroup is OOM killed due to exceeding its memory.max
> > > > limit, and all the tasks in the cgroup are trying to exit simultaneously.
> > > >
> > > > When this happens, it can sometimes take hours for tasks to exit,
> > > > as they are all trying to squeeze things into zswap to bring the group's
> > > > memory consumption below memory.max.
> > > >
> > > > Allowing these exiting programs to push some memory from their own
> > > > cgroup into swap allows them to quickly bring the cgroup's memory
> > > > consumption below memory.max, and exit in seconds rather than hours.
> > > >
> > > > Signed-off-by: Rik van Riel <riel@surriel.com>
> > >
> > > Thanks for sending a v2.
> > >
> > > I still think maybe this needs to be fixed on the memcg side, at least
> > > by not making exiting tasks try really hard to reclaim memory to the
> > > point where this becomes a problem. IIUC there could be other reasons
> > > why reclaim may take too long, but maybe not as pathological as this
> > > case to be fair. I will let the memcg maintainers chime in for this.
> > >
> > > If there's a fundamental reason why this cannot be fixed on the memcg
> > > side, I don't object to this change.
> > >
> > > Nhat, any objections on your end? I think your fleet workloads were
> > > the first users of this interface. Does this break their expectations?
> >
> > Yes, I don't think we can do this, unfortunately :( There can be a
> > variety of reasons for why a user might want to prohibit disk swap for
> > a certain cgroup, and we can't assume it's okay to make exceptions.
> >
> > There might also not *be* any disk swap to overflow into after Nhat's
> > virtual swap patches. Presumably zram would still have the issue too.
>
> Very good points.
>
> >
> > So I'm also inclined to think this needs a reclaim/memcg-side fix. We
> > have a somewhat tumultous history of policy in that space:
> >
> > commit 7775face207922ea62a4e96b9cd45abfdc7b9840
> > Author: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> > Date:   Tue Mar 5 15:46:47 2019 -0800
> >
> >     memcg: killed threads should not invoke memcg OOM killer
> >
> > allowed dying tasks to simply force all charges and move on. This
> > turned out to be too aggressive; there were instances of exiting,
> > uncontained memcg tasks causing global OOMs. This lead to that:
> >
> > commit a4ebf1b6ca1e011289677239a2a361fde4a88076
> > Author: Vasily Averin <vasily.averin@linux.dev>
> > Date:   Fri Nov 5 13:38:09 2021 -0700
> >
> >     memcg: prohibit unconditional exceeding the limit of dying tasks
> >
> > which reverted the bypass rather thoroughly. Now NO dying tasks, *not
> > even OOM victims*, can force charges. I am not sure this is correct,
> > either:
> >
> > If we return -ENOMEM to an OOM victim in a fault, the fault handler
> > will re-trigger OOM, which will find the existing OOM victim and do
> > nothing, then restart the fault. This is a memory deadlock. The page
> > allocator gives OOM victims access to reserves for that reason.
> >
> > Actually, it looks even worse. For some reason we're not triggering
> > OOM from dying tasks:
> >
> >         ret = task_is_dying() || out_of_memory(&oc);
> >
> > Even though dying tasks are in no way privileged or allowed to exit
> > expediently. Why shouldn't they trigger the OOM killer like anybody
> > else trying to allocate memory?
>
> This is a very good point and actually out_of_memory() will mark the
> dying process as oom victim and put it in the oom reaper's list which
> should help further in such situation.
>
> >
> > As it stands, it seems we have dying tasks getting trapped in an
> > endless fault->reclaim cycle; with no access to the OOM killer and no
> > access to reserves. Presumably this is what's going on here?
> >
> > I think we want something like this:
>
> The following patch looks good to me. Let's test this out (hopefully Rik
> will be able to find a live impacted machine) and move forward with this
> fix.

I agree with this too. As Shakeel mentioned, this seemed like a
stopgap and not an actual fix for the underlying problem. Johannes
further outlined how the stopgap can be problematic.

Let's try to fix this on the memcg/reclaim/OOM side, and properly
treat dying tasks instead of forcing them into potentially super slow
reclaim paths. Hopefully Johannes's patch fixes this.